The nitride semiconductor crystal refers to gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), boron nitride (BN) and a mixture of those crystals (hereinafter called a mixed crystal). The growth of a crystal of a nitride semiconductor is carried out on a substrate of sapphire (Al2O3) or silicon carbide (SiC) having a hexagonal basic crystal structure and a lattice constant close to it. In order to perform the growth of a crystal with high quality, it is necessary to control the concentration of impurities and the thickness of a grown film strictly.
Molecular beam epitaxy (hereinafter MBE) and vapor-phase epitaxy (hereinafter VPE) are mainly available as the methods for the growth of a nitride semiconductor crystal. VPE includes one in which a material gas to be epitaxially grown flows horizontally relative to s substrate surface (hereinafter horizontal) and one in which it flows vertically (see, for example, JP-A-2000-216101, JP-A-2001-302398, JP-A-2000-49104 and JP-A-2004-47763).
The parameters which require control for epitaxial growth by VPE include substrate temperature, Group 5 material gas/Group 3 material gas feed ratio (hereinafter 5/3 ratio), total amount of gases flowing in the reaction tube (=Group 3 gas+Group 5 gas+carrier gas), internal pressure of the reaction tube and flow rate of gases in the reaction tube, and the adjustment of those parameters makes it possible to control the crystal film thickness, crystal quality, etc.
While a grown film is still very thin relative to the substrate when crystal growth is performed as stated above, it is arranged with a one to one correspondence with an atom in the substrate and their difference in lattice constant is of little importance. When AlN is, for example, grown on SiC, AlN is arranged with the atomic spacing which SiC has. However, as the grown film becomes thicker, it is arranged in accordance with its own lattice constant and in its interface, there normally occur portions failing to have a one to one correspondence with the atoms in the substrate. In other words, lattice defects, such as dislocations, occur and their one to one correspondence is lost. The critical thickness of the film at which dislocations occur is called the critical film thickness.
As a method for the epitaxial growth of compound semiconductor crystals composed of elements of Groups 3 and 5, such as nitride semiconductor crystals, there are a method in which elements of Groups 3 and 5 are simultaneously fed to the substrate in the beginning of growth (hereinafter simultaneous feeding method) and a method in which they are alternately fed (hereinafter alternate feeding method).
It is known that when AlN is epitaxially grown over a thickness causing lattice relaxation (critical film thickness) on the (0001)Si plane of an SiC substrate by using the simultaneous feeding method in a horizontal VPE apparatus, an epitaxial layer of AlN is improved in flatness if the growth parameters are appropriately selected.
On the other hand, the alternate feeding method is a method carried out to keep accurately the periodicity of Group 3 atoms/Group 5 atoms which the compound semiconductor crystal structure has. The alternate feeding method employing a substrate of gallium arsenide is disclosed in literature (Yoshiji Horikoshi, et al., “Surface atomic processes during flow-rate modulation epitaxy”, Appl. Surf. Sci., 112, pp.48-54 (1997); Masanobu Hiroki, et al., “Flat Surface and Interface in AlN/GaN Heterostructures and Superlattices Grown by Flow-Rate Modulation Epitaxy”, Jpn. J. Appl. Phys., Vol. 42 (2003), pp. 2305-2308).
As a known example employing heterojunction and the simultaneous feeding method, there is a case in which a nitride semiconductor, GaN, is grown directly in a highly limited area on an SiC substrate (hereinafter selective growth) by utilizing the characteristics of the heterojunction between the nitride semiconductor and the SiC substrate. An example of publication of a heterojunction device is described in literature (John T. Torvik, et al., “A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300° C.”, MRS Internet J. Nitride Semicond. Res. 4, 3 (1999)).
As a known example employing a buffer layer without forming a nitride semiconductor directly on a heterogeneous substrate, there is a case in which a nitride semiconductor epitaxial layer is used as an optical device (e.g., a blue light-emitting diode). Epitaxial growth in the case in which only a nitride semiconductor is applied as a functional device is effected on that surface of a single-crystal sapphire or SiC substrate on which a buffer layer is formed from a material of the same composition as the nitride semiconductor. The buffer layer is, for example, a film of a nitride semiconductor formed at a low substrate temperature, or one having a superlattice structure of GaN and AlN.
For the epitaxial growth of a nitride semiconductor of high quality, it is most desirable to use a single-crystal substrate of a nitride semiconductor coinciding with it perfectly in lattice constant. However, as no such single-crystal substrate of high quality exists, it is actually the case to use instead a substrate of sapphire or SiC having the same hexagonal crystal system with the nitride semiconductor. Differences in physical constants occur between the substrate and the nitride semiconductor as stated below. More specifically, there occur a difference in lattice constant, a difference in coefficient of thermal expansion and a difference in surface energy.
The differences in physical constants as stated above present a common problem of worsening in crystallinity which occurs as stated below when the epitaxial growth of a nitride semiconductor is effected directly on the substrate.
Firstly, when the critical film thickness is not exceeded, the nitride semiconductor crystal is epitaxially grown with strain to adapt itself to the lattice constant of the substrate, but when it is exceeded, the nitride semiconductor crystal tends to grow epitaxially in accordance with its own lattice constant and the resulting problem is that the atoms in the substrate and the nitride semiconductor do not correspond to each other on a one to one basis, but lattice defects, such as dislocations, occur in the interface. The lattice defects which have occurred extend through the epitaxial layer as they are.
There is also a problem of strain produced in the epitaxial layer with a lowering of the temperature for epitaxial growth to room temperature. This is presumably due to the stress created in the layer by the difference in coefficient of thermal expansion between the substrate and the film deposited thereon. The epitaxial layer may crack, depending on the kind and thickness of the nitride semiconductor.
Another problem occurring from the direct epitaxial growth of a nitride semiconductor on a sapphire or SiC substrate by the simultaneous feeding method based on VPE is that in the region of its interface with the substrate, the nitride semiconductor does not grow in layers but in islands as a result of an intermediate reaction between feed gases (e.g. TMA and NH3). The continuity of the crystal is lost between islands, leading to a lattice defect remaining in the nitride semiconductor in the interface of junction, or a roughened surface as no flat epitaxial layer can be obtained. Since an overlying layer is formed on an underlying island layer serving as a template, it grows with its defect in flatness.
Examples in which the alternate feeding method is presently carried out are all directed to surfaces lying over 3-5 compound semiconductor layers, and there is no epitaxial growth effected directly on the surface of a sapphire or SiC substrate. There has not been known any example in which good results have been obtained from heteroepitaxial growth effected directly on a heterogeneous substrate by using the alternative feeding method.
There is a case in which a nitride semiconductor, GaN, is grown directly in a highly limited area on an SiC substrate, as described in the above-cited literature (John T. Torvik, et al., “A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300° C.”, MRS Internet J. Nitride Semicond. Res. 4,3 (1999)). This is, however, unsatisfactory in device performance, including a very low ratio of current amplification, due to the poor electrical characteristics of the heterojunction (interface) between GaN and SiC. Accordingly, there is a demand for the establishment of a method of growing a flat nitride semi-conductor crystal of high quality exhibiting improved electrical characteristics in its heterojunction and capable of playing a satisfactory role as a device.
In known examples of structures in which a buffer layer is formed between a nitride semiconductor layer and a substrate, the buffer layer turns the interface between the substrate and the heterojunction into a region in which defects and strain are concentrated, sacrificing the crystal.
The epitaxial growth of a nitride semiconductor by normal pressure VPE is also defective in that an atmospheric pressure prevailing in the reaction tube causes disorder in the flow of material gases in the vicinity of a heated substrate and makes it impossible to realize epitaxial growth in layers.